For this month's topic, I decided to look higher than usual and outside the Earth's atmosphere at the solar wind, which has a major role in space weather and the occurrence of the auroras on Earth.
The American Meteorological Society Glossary defines wind as: “Air in motion relative to the surface of the earth.” In this installment of Weather Almanac, I will tweak that definition for one appropriate for the solar wind: “Plasma in motion relative to the solar system.” That is, the continuous flow of charged particles outward from the Sun that permeates the solar system. We still need the “relative” part since both the Earth and the Solar System are moving through space at a high velocity, and rotating as well.
The Solar Wind
The solar wind is a stream of highly energized, charged particles, primarily electrons, protons and heavier ionized atoms, that moves outward from the Sun at speeds as high as 1000 km/s ( 2,240,000 mph) and at a temperature of 1 million degrees (Kelvin). The solar wind arises when the hot outermost layer of the solar atmosphere, the solar corona, expands into space. (It is the corona that is visible shining around the solar disk during a solar eclipse.) The corona's high temperature moves the particles (plasma) to such a speed that the Sun's gravity cannot hold on to them and they escapes.
Like its earthly counterpart, the solar wind does not blow steadily in time or speed, but eddies and gusts as it is ejected out of the solar mass and travels through the solar system. The one consistent aspect is that essentially it always moves outward from the Sun. The solar wind routinely ebbs and flows through the Sun's 27-day rotation, as well as sporadically changing strength, in response to violent eruptions in the corona (coronal mass ejections or CMEs).
Plot of Solar Wind Variation Image Courtesy NASA/Marshall Space Flight Center
Day after day, year after year, our Sun flings out these high-energy streamers of ionized gas at the rate of about 1 million tonnes of matter every second. The solar wind sweeps particles and their magnetic fields across interplanetary space toward Earth at supersonic speeds. Its average speed of 400 km/s (895,000 mph), but varies in speed from as high as 800 km/s (1,790,000 mph) over coronal holes and as low as 300 km/s (670,000 mph) over streamers. The wind can contain magnetic clouds in which the plasma composition varies. Where fast and slow wind streams interact, a buffeting solar wind gusts like a terrestrial wind stream.
One interesting aspect of the solar wind is that it pushes a comet's tail away from the body as it travels through the solar system. Indeed, the presence of the solar wind can be deduced by observing the tails of comets. When the comet's particles are caught in the stream of the outgoing solar wind, they are pushed outward into the solar system. Thus, the tail always points away from the sun. When a comet is headed toward the Sun, the tail points backward, away from the sun. But when it is moving away from the sun, its tail precedes the comet.
Like the terrestrial wind, the solar wind can aid or retard objects moving through it. (In addition to any electrical and magnetic effects.) One proposed method of fuelless travel to the outer planets and perhaps the stars considers the use of a solar sail to catch this solar wind, making the spacecraft a stellar sailing craft.
The Solar Wind and The Earth
The solar wind interacts with magnetic fields around any of the planets as it passes. The resultant interaction distorts the field, with a loose analogy to a rock in a rushing stream.
The Solar Wind and the Earth-Space Environment: Art Work Courtesy of NASA
As the solar wind passes the Earth, it interacts with and distorts the Earth's magnetic field. The wind compresses the field in toward the Earth on the sunward side and stretches it out in the anti-sun direction. This gives the Earth's magnetosphere a shape similar to a comet and its tail.
The magnetosphere is a region of rarefied, ionized gases caught in the Earth's magnetic field and located from 150 km (93 miles) to 70,000 km (42,000 miles) in altitude on the sunward side of earth and out to 300,000 km (184,000 miles) on the side of the planet away from the sun. A gusty, stormy solar wind will causes extraordinary variations in Earth's magnetic field, producing rapid changes in its direction and intensity. These conditions are now known by the name of space weather.
The Earth's Magnetic Field Forms the Magnetosphere (Credit NASA)
Within a region of the magnetosphere called the magnetopause, the Earth's magnetic field dominates the effects of the solar wind. The small fraction of the charged particles which do leak through the magnetopause are trapped in two large doughnut-shaped rings called the Van Allen radiation belts.
Variations in space weather result from the blowing and gusting of the solar wind, primarily from changes in the speed or density of the wind. When solar storms rage on Sol's surface, as they do during solar cycle maxima, they send out large and active bursts of particles (and associated magnetic fields) toward Earth. The main feature on the Sun's surface that causes such solar storms is sunspot activity.
Aurora Borealis over Finland, Russia, Estonia and Latvia Image from Expedition 12 crew aboard the International Space Station Courtesy NASA
When a solar wind storm buffets the magnetosphere and portions of its energy are transferred to the magnetosphere, a geomagnetic storm results on Earth. Some electrons in the solar wind become trapped in the Earth's magnetic field and are then accelerated toward the magnetic polar regions and down into the upper atmosphere. Here, high in the atmosphere, the solar wind interaction produces the incredible beauty of the aurora borealis around the northern magnetic pole and the aurora australis around the southern pole. (For more on the aurora and space weather, see my articles by clicking on the names.)
Proposal and Discovery
Although the solar wind was not confirmed until the age of space exploration began, its existence had been postulated as early as 1859 by British astronomer Richard C. Carrington. He based his hypothesis on the observation of a solar flare, the first to be scientifically documented, and magnetic activity on Earth the following day. This large geomagnetic storm induced the largest known geomagnetic response know to that time. Auroral activity extended as far south as the Caribbean in the Northern Hemisphere, and the disturbance affected telegraph service across Europe and North America.
Irish physicist George Fitzgerald would later suggest that the Sun emitted streams of matter from its body into the solar system. A century or so later (1959), that hypothesis was confirmed by the Soviet space probe Luna 1 that measured the strength of the solar wind. The findings were confirmed by Luna 2 and Luna 3, Venera 1 and the American probe Mariner 2.
Artist concept of NASA's Voyager spacecraft. Image credit: NASA/JPL-Caltech
In the following half century, many studies have been made from spacecraft as well as surface observations of the solar wind --- including its furthest extent. The now far-distant probe Voyager 1 observed on 13 December 2010 that the velocity of the solar wind at its location, 17.3 billion km (10.8 billion miles) from Earth, had slowed to zero. Dr. Edward Stone, the Voyager project scientist remarked "We have gotten to the point where the wind from the Sun, which until now has always had an outward motion, is no longer moving outward; it is only moving sideways so that it can end up going down the tail of the heliosphere, which is a comet-shaped-like object." (More recent (2012) data from Voyager 1 suggests the craft may be in the transition zone between the solar system and interstellar space.)
Look Up, Way Up
The year 2013 was predicted to have been the peak of the current 11 year sunspot cycle that would generate more space storms directed toward Earth. But a recent article from NASA noted that the number of sunspots has actually decreased below the 2011 numbers. They suggest the Sun could be in the valley of a twin peaking of solar activity as happened in 1998 and 2001. In either case, it is still a time of higher solar activity, and thus we should be on the watch for more frequent auroral activity over the next year thanks to the solar wind.
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